A power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is used for electric power conversion in consumer electric appliances and automobile motors or the like, and typically provided with electrodes both on an upper face and on a bottom face. Since this type of semiconductor device is used for power electric applications, it is required to sustain higher voltage, and to reduce power consumption. Thus, it is necessary to reduce resistance, when operating in the ON state (in the following, referred to as ON resistance).
A field plate structure is used as one of the ways for reducing the ON resistance. The field plate structure includes a gate electrode provided in a trench via a gate insulating film and a field plate electrode provided below the gate electrode via a field plate oxide film in the trench. Since the field plate electrode enhances carrier depletion in the drift layer, it is possible to reduce the specific resistance of a drift layer, while sustaining a high voltage. Hence, it is advantageous to reduce the ON resistance in this type of the MOSFET.
On the other hand, the distance between the trenches becomes narrower as miniaturizing the power MOSFET, resulting in the smaller width of a base region sandwiched therebetween. Accordingly, it is difficult to form a source region and a carrier extraction region in the base region, because alignment and patterning in the photolithography become too tight and fine.